1. Field of the Invention
The present invention relates to a Fourier transform infrared spectrophotometer (FTIR) designed to control a movable mirror by means of a quadrature control. The FTIR is usable for qualitative and quantitative analyses of a sample and adaptable to a wide range of substances including polymeric and semiconducting materials, irrespective of whether they are organic or inorganic.
2. Description of the Related Art
Generally, a Fourier transform infrared spectrophotometer (FTIR) comprises a main interferometer section for performing a sample measurement operation, and a control interferometer section for instructing the main interferometer section to start data acquisition, and stabilizing a sliding speed of a movable mirror in the main interferometer section. In the control interferometer section, a control system called “quadrature control” is employed to calculate a position of the movable mirror. Specifically, the quadrature control system comprises a phase plate, such as a λ/8 plate, disposed between a beam splitter and a fixed mirror, a polarization beam splitter operable to separate P and S waves from an interference signal combined through the beam splitter, and a detector operable to detect the separated P and S waves, whereby a position of the movable mirror can be calculated from a phase relationship between detection signals indicative of the P and S waves detected by the detector, and respective wavenumbers of the detection signals.
The FTIR also includes a movable-mirror sliding mechanism operable to move the movable mirror close to and away from the beam splitter so as to change a position of the movable mirror. FIGS. 6A and 6B show one example of the movable-mirror sliding mechanism together with the movable mirror swingably hung thereon, wherein FIG. 6A is a vertical sectional view, and FIG. 6B is a left side view. The movable mirror 4 is attached to a base 131 through a mirror holder 134, and the base 131 is swingably attached to two plates 132, 133, each which is swingably attached to a top portion of a body 130 by films 141, 142. The body 130 internally has a magnet 136 and a pole piece 137 which are fastened by a bolt 138 to a plate 135 fixed to the body 130, and associated with a coil 139 fixed to the base 131 through an angle rest 140. The coil 139 is adapted, in response to applying a current thereto, to be moved across a magnetic field formed by the magnet 136 and the pole piece 137. More specifically, when a current is applied to the coil 139, the coil 139 is subjected to a Lorentz force according to a magnetic field from the magnet 136 and the pole piece 137 and thereby moved in a rightward/leftward direction in FIG. 6A. Consequently, the movable mirror 4 is moved in the rightward/leftward direction in FIG. 6A through the base 131. In this manner, the movable mirror 4 is moved close to and away from the beam splitter to change a position thereof relative to the beam splitter.
In the FTIR having the movable-mirror sliding mechanism as shown in FIG. 6, if a mounting base of the body 130 improperly inclines, respective distances between the movable mirror 4 and the beam splitter and between the fixed mirror and the beam splitter will become different from each other when the movable mirror 4 is located at a gravitationally balanced position (i.e., an initial position of the movable mirror 4 as shown in FIG. 6A), while, on the other hand, a center burst which has a maximum intensity value in an interferogram obtained by moving the movable mirror 4 during an adjustment operation will be detected when the above two distances become equal to each other, i.e., at a position in the interferogram deviated from that corresponding to the gravitationally balanced position. FIG. 7 shows one example of the interferogram in the above situation, wherein the horizontal axis represents a position of the movable mirror 4, and the vertical axis represents an intensity value. During a sample measurement operation after the above adjustment operation, the movable mirror 4 is moved in a moving range having a center set at the detected position (−L) of the center burst (hereinafter referred to as “center burst position”).
The operations of detecting a center burst position and measuring a sample in the conventional FTIR will be more specifically described below. FIG. 8 is an explanatory diagram of these operations, wherein a position of the movable mirror 4 is indicated in a lateral (rightward/leftward) direction, and the rightward and leftward directions are defined as (+) and (−) directions, respectively.
First of all, the movable mirror 4 is set at a gravitationally balanced position (1), and positional data of the movable mirror 4 is initialized. Subsequently, the movable mirror 4 is slidingly moved from the gravitationally balanced position (1) to a position (3) by a distance corresponding to a wavenumber resolution of 2 cm−1 (about −2.5 mm; the number of measurement points=8192 points) as indicated by an arrow (2), and then the sliding direction is reversed. Then, the movable mirror 4 is slidingly moved over a moving range (4) for obtaining a wavenumber resolution of 2 cm−1 (about 5 mm; the number of measurement points=16384 points), and an operation of acquiring data for a center burst position is performed during a period of this sliding movement. A position where an interferogram obtained in the moving range (4) has a maximum intensity value is calculated and detected as a center burst position (−L). Then, when the movable mirror 4 is located at a position (6), a measurement start position is calculated to allow a sample measurement operation to be performed in a moving range symmetrically about the detected center burst position (−L) with a designated wavenumber resolution. If the measurement start position is determined as a position (7), the movable mirror 4 is slidingly moved to the position (7), and then the sliding direction is reversed to perform a 1st operation of acquiring sample data in a moving range (8). Subsequently, 2nd to n-th operations of acquiring sample data will be sequentially performed in the moving range (8) having a measurement start position set at the position (7).
As above, a position having a maximum intensity value in an interferogram obtained by moving the movable mirror 4 in a predetermined moving range having a center set at the gravitationally balanced position (1) is detected as a center burst position (−L). However, if a surface of a support base mounting the FTIR thereon largely inclines relative to a proper position, the center burst position (−L) will be largely deviated from a position in the interferogram corresponding to the gravitationally balanced position (1) to cause difficulty in accurately detect the center burst position (−L). Although the moving range of the movable mirror 4 may be pre-set at a larger value to avoid the above situation, this approach involves a problem about an increase in time for the adjustment operation of moving the movable mirror 4 to detect a center burst position (−L), particularly, when considering that the adjustment operation has to be performed every time a sample is changed. With a view to solving this problem, there has been disclosed an FTIR designed to perform a pre-adjustment operation of moving a movable mirror 4 in a relatively wide moving range to detect a center burst position (−L) and storing the detected center burst position in a storage device, such as a nonvolatile memory, and then, in a sample measurement operation, initially perform a fine-adjustment operation of moving the movable mirror 4 in a relatively narrow moving range having a center set at the stored center burst position so as to finely adjust the center burst position (see, for example, Japanese Patent No. 2858630).
In reality, during an operation of detecting a center burst position (−L), a light intensity reaching a detector is likely to become insufficient, depending on a measurement technique (e.g., reflection measurement) or a type of sample. As a result, an interferogram will have a low intensity at a center burst position (−L) as shown in FIG. 9. This means that a center burst position (−L) is liable to be buried by noises, or a noise peak is liable to be erroneously recognized as a center burst.